The present invention relates to a technique for controlling sound image localization.
A technique in which, as shown in FIG. 8(a), for example, volumes of sounds emitted respectively from speakers SP1 and SP2 that are placed in front of the listener LP are adjusted to localize the sound image at a position on the straight line connecting the speakers is known as panning. A transfer function (head transfer function) simulating the transfer characteristics of sounds which reach the right and left ears of the listener from a position (virtual sound source position) that is different from the installation positions of the speakers SP1 and SP2 of FIG. 8(a) is convoluted with audio signals to be supplied respectively to the speakers SP1 and SP2, thereby causing the sound image to be localized at the virtual sound source position so that auditory sensation in which the sound source exists at the virtual sound source position is perceived by the listener. Hereinafter, a sound source which is localized at a virtual sound source position is referred to as “virtual sound source”, and that which actually exists at the position of a speaker is referred to as “real sound source”. Patent References 1 and 2 disclose a technique in which a virtual sound source is treated in the same manner as a real sound source, and a sound image is localized at a position on the straight line connecting the virtual sound source and the real sound source by distributing signal components between the virtual sound source and the real sound source (adjustment of the gain: panning), thereby causing the sound image to be localized in an arbitrary direction including the height direction.    (Patent Reference 1) Japanese Patent No. 4,306,029    (Patent Reference 2) JP-A-6-303699    (Patent Reference 3) JP-A-2007-288677    (Patent Reference 4) JP-A-8-205297    (Patent Reference 5) Japanese Patent No. 4,567,049    (Patent Reference 6) Japanese Patent No. 3,368,835
However, the technique disclosed in Patent References 1 and 2 is considered to have a limitation that, when a sound image is to be localized at a position on the straight line connecting the virtual sound source and the real sound source, an audio signal corresponding to the virtual sound source and that corresponding to the real sound source cannot be supplied to the same speaker, because of the following reason.
FIG. 8(b) is a view showing an example of the signal process in the technique disclosed in Patent Reference 1. More specifically, FIG. 8(b) is a view showing an example of the signal process in the case where audio signals (hereinafter, referred to as virtual-sound source signals) XV1 and XV2 which cause the virtual sound source VSS to be localized through sounds emitted from the speakers SP1 and SP2 in FIG. 8(a), and an audio signal XR which drives the speaker SP1 as the real sound source RSS1 (i.e., an audio signal indicative of a sound emitted from the position of the speaker SP1: hereinafter, referred to as a real-sound source signal) are produced from the same input audio signal X, and a sound image SI is localized at a position on the straight line connecting the virtual sound source VSS and the real sound source RSS1, by performing signal distribution between the real-sound source signal and the virtual-sound source signals.
In the signal process shown in FIG. 8(b), the audio signal to be supplied to the speaker SP1 is produced by adding the real-sound source signal XR and the virtual-sound source signal XV1 together in an adder 30. As shown in FIG. 8(b), the real-sound source signal XR is a signal which is obtained by applying a gain adjustment by a gain controller 10r to the input audio signal X. On the other hand, the virtual-sound source signals XV1 and XV2 are obtained by applying a gain adjustment by a gain controller 10v to the input audio signal X, and then performing a convolution of the head transfer function H by a virtual-sound source processing section 20.
It seems that, in order to localize the sound image SI at a position on the straight line connecting the virtual sound source VSS and the real sound source RSS1, in a similar manner as the sound image localization by signal distribution between real sound sources, the gains Cr and Cv of the gain controllers 10r and 10v are adequately adjusted so as to satisfy following Exp. (1):0≦Cr≦1,0≦Cv≦1, and (Cr)2+(Cv)2=1  (1)
However, the real-sound source signal XR and virtual-sound source signal XV1 which are obtained by the signal process shown in FIG. 8(b) are obtained from the same audio signal (in the example shown in FIG. 8(b), the input audio signal X), the amplitudes and phases of the two signals have constant relationships, and the signals are highly correlated with each other. When the real-sound source signal XR and the virtual-sound source signal XV1 are added together by the adder 30, therefore, the signals are mixed with each other, a sound which is obtained by applying a filter process with filter characteristics of (Cr+CvH) to the audio signal X is output from the speaker SP1, and a sound is just heard in such a manner that the virtual sound source VSS is localized at an unexpected position, or that the listener LP hears a sound in which the frequency characteristics are deteriorated, as if the sound is emitted from the speaker SP1, with the result that the virtual sound source VSS is not localized at an expected position. Since the virtual sound source VSS is not localized at an expected position, also localization of the sound image SI is disturbed.
Usually, at least two speakers are used in localization of a virtual sound source. In the case where, for example, front right and left, and rear right and left, or a total of four speakers are used, therefore, it is possible also to avoid overlapping of speakers to which real-sound source signals are output, and those to which virtual-sound source signals are output. Specifically, in the case where virtual-sound source signals are output to the rear right and left speakers, real-sound source signals are supplied to the front right and left speakers, and, in the case where virtual-sound source signals are output to the front right and left speakers, real-sound source signals are supplied to the rear right and left speakers. In this mode, however, virtual-sound source processing sections must be separately disposed in the sets of the front right and left speakers, and the rear right and left speakers, respectively, thereby causing a problem that the configuration of an audio apparatus is complicated.